1.Field of the Invention
This invention relates to a wide angle objective lens, and particularly to a wide angle objective lens suitable for use with a 35 mm format lens shutter camera, a 35 mm format camera with a range finder or the like.
2. Related Background Art
An optical system of four-unit construction of the so-called telephoto type in which a posterior unit having negative refractive power is disposed rearwardly of an anterior unit of positive-negative-positive lens unit construction having positive refractive power to shorten the full length of a standard lens for a compact camera to the focal plane thereof is known as is disclosed, for example, in Japanese Patent Publication No. 44-10831.
As a wide angle objective lens of such telephoto type and having an angle of view wider than that of a standard lens, there are known an example in which as disclosed in U.S. Pat. No. 4,394,073, an anterior unit disposed forwardly of a negative posterior unit is made into a four-unit construction of positive-negative-positive-positive to thereby improve coma and the eclipse of the marginal light flux, an example in which as disclosed in U.S. Pat. No. 4,235,521, a negative posterior unit is comprised of two negative and positive meniscus lenses to thereby achieve an angle of view of 60.degree. or greater and brightness of F-number 2.8, an example in which as disclosed in Japanese Laid-Open Patent Application No. 1-209413, a negative lens component in an anterior unit is a cemented lens comprising a negative biconcave lens and a positive meniscus lens, etc..
In these optical systems of the telephoto type, however, in order to achieve the compactness thereof, the refractive powers of the anterior unit having positive refractive power and the posterior unit having negative refractive power have been made great and moreover, the air gap between the anterior unit and the posterior unit has been made great. Therefore, the marginal light flux has been eclipsed, and this has led to the disadvantages that not only the quantity of marginal light becomes deficient, but also distortion shifts greatly to the positive and chromatic difference of magnification also is greatly fluctuated by the angle of view. Further, the refractive power of each lens component is great, and this has led to the disadvantage that curvature of image field increases.
Also, in the aforementioned U.S. Pat. No. 4,235,521, a positive lens is disposed most adjacent to the image side and this has led to the disadvantage that the effecitve diameter of this positive lens most adjacent to the image side becomes great and further, from the mutual ecccentricity or inclination or the like of the negative lens and the positive lens in the posterior unit, it has been difficult to provide a predetermined optical performance in manufacture. Also in the wide angle lens described in the aforementioned U.S. Pat. No. 4,394,073, from the mutual eccentricity or inclination or the like of a negative second lens component and a positive third lens component, it has been difficult to obtain a predetermined optical performance in manufacture.
Further, in the wide angle lens described in the aforementioned Japanese Laid-Open Patent Application No. 1-209413, the cemented surface in a negative second lens component has negative refractive power and therefore, positive spherical aberration of high order has occurred and the correction of spherical aberration could not be sufficiently accomplished.